It is well known that when many ferrous-based low carbon steels and other metals are exposed to moisture and air they rust, corrode or otherwise degrade. When these low carbon steels are used to reinforce concrete or other building materials, they frequently become exposed to harsh weather and brine from road salts or the sea which greatly accelerate this corrosion and accordingly greatly reduce the life span of the reinforced elements or roadway. The Department of Transportation of the State of Florida estimates that this damage to bridge decks and associated support structures alone imposes a repair cost of from 160 to 500 million dollars anually in the U.S. alone.
The process underlying this deterioration of the roadway is the deterioration and volume expansion of the reinforcing steel element. This in turn is caused by the conversion of the iron (Fe) within the low carbon steel to iron oxide--rusting.
The underlying corrosion is caused by the formation of electrolytic cells in which different parts of the steel reinforcement can be both the anode and the cathode of an electrolytic cell when in the presence of moisture. Differences in ionic concentrations at different sites determine whether a particular site is cathodic or anodic. Anodic reaction products (i.e. iron oxide) have a greater volume than the original steel, so great internal pressure is applied within the concrete, causing it to crack and spall.
Cathodic protection is one method recognized as being useful in preventing or retarding the process. It involves introducing a separate additional electrode (anode) and applying an impressed current such that all of the steel reinforcement becomes cathodic, thus preventing the formation of iron oxide.
Many cathodic protection systems are known. For example, U.S. Pat. No. 3,868,313, issued to P. J. Gay, Feb. 25, 1975, discloses a cathodic protection system comprising applying an electrically insulating coating on the substrate followed by the application of an electrically conductive coating over the insulating coating. A D.C. voltage is then applied between the metal substrate and the conductive coating.
U.S. Pat. No. 3,151,050, issued Sept. 19, 1964, discloses methods for cathodic protection for vehicles and components in storage. The method comprises the application of an electrically conductive paint to the metal to be protected. The paint is a suspension of carbon, manganese dioxide, ammonium chloride and an organic filler and a solvent such as methyl-ethyl-ketone. A second coating of resin containing metallic copper is then applied, followed by a final coat of paint or enamel. Lastly, a D.C. voltage is applied between the conducting paint and the metal base.
Other polymer compositions containing various carbon based materials are also known. U.S. Pat. No. 4,035,265, issued July 12, 1977, to J. A. Saunders discloses electrically conductive paint compositions employing graphite and colloidal carbon. The graphite is subjected to wet grinding so as to reduce the graphite to thin platelets. The colloidal carbon employed consists of particles having a size from 20 to 50 millimicrons. The final composition (including the article it is applied to) is used as a heat source when electrical current is passed through the coating.
Other efforts at carbon-containing coatings are found in
(1) U.S. Pat. No. 3,505,263, which discloses finely divided calcined petroleum coke in a polymer latex binder;
(2) U.S. Pat. No. 3,404,019, which discloses the use of fluid petroleum coke as a filler or pigment in polymeric compositions;
(3) U.S. Pat. No. 2,730,597, which discloses resistance elements which optionally employ various materials in a resin base;
(4) U.S. Pat. No. 4,476,265, which discloses poly (arylene sulfide) compositions which contain a "black carbonaceous pigment";
(5) U.S. Pat. No. 4,444,837, which discloses coating or sealing-type plastisols which contain carbon dust as a filler;
(6) U.S. Pat. No. 3,391,103, which discloses phenolic resin compositions which employ "oxidized carbon particles";
(7) U.S. Pat. No. 3,615,754, which discloses an ink which employs 2 to 10 percent of ground coke; and
(8) U.S. Pat. No. 3,444,183, which discloses a film forming composition made from a heat-resistant polymer and a dispersion of carbon particles.